I would ask for about 3x more for a wedding then for an evening of dancing:

* weddings are a lot of hassle

* you're only playing/calling for a short time, but you need to be there very early and schedules are not reliable at weddings -- this may be worse for bands than callers

* people spend so much at most weddings these days that they're typically willing to pay whatever (once I let someone negotiate me down a lot by them saying they were trying to save money and when I got therei found out that the food cost more per guest then they were paying each of us)

* I'd charge much less or free for a friend, especially if I would be at their wedding anyway: it's like a wedding gift